The present invention relates generally to high current, charged particle accelerators, and more particularly to high current, betatron accelerators.
High current, high energy charged particle beams have a variety of potential applications in, for example, directed energy systems, in the excitation of free electron lasers, and in fusion-related applications. Over the last fifty years, several charged particle cyclic accelerators have been developed that are capable of generating charged particle beams with energies in excess of 10 MeV. The most prominent of these accelerators are the betatron, synchrontron, and the microtron accelerators. However, the peak current of these accelerators is usually under one ampere. This limit on the current is imposed primarily by the space charge of the beam that either makes the orbits of the gyrating particles unstable or acts to drive catastrophic collective instabilities.
The present invention is directed toward a modified betatron design. Betatron accelerators generally comprise an annular or toroidal vacuum chamber with a time-varying magnetic field, referred to as the betatron field, disposed relative to the toroidal chamber for the purpose of accelerating charged particles injected in the chamber. Typically, the strength of this betatron field is designed to be maximal along the major axis of the toroid and is designed to decrease with radial distance. Accordingly, electrons injected tangentially are accelerated at a constant radius by this betatron field into a privileged orbit known as the "betatron orbit".
Typically, the current and energy of the charged particle beam injected into the toroidal chamber is low such that the ratio of the Budker parameter (.nu.) to the relativistic factor (.gamma.) is on the order of 10.sup.-5. Injection at these low energies results in most of the charged particles of the injected beam striking the walls of the toroidal chamber during the first revolution within the toroid. This beam loss can be understood through the following equation relating the major radius of rotation r of the beam to the injected energies. EQU r=v/(eB.sub.z /mc.gamma.),
where v is the velocity of the particle, B.sub.z is the betatron field, m is the mass of the injected particle, and .gamma. is the relativistic factor that is proportional to the beam energy. From this equation, it can be seen that as the relativistic factor (.gamma.) becomes smaller in order to provide energy to build-up the fields inside the vacuum chamber, the major radius of revolution r will also become smaller. Thus, as the beam of charged particles loses energy and .gamma. decreases, the radius will decrease and the individual charged particles in the beam will strike the walls of the toroid. A variety of different techniques have been formulated to prevent this loss of charged particles to the chamber walls. However, none of these techniques have been entirely successful and thus the trapping efficiency of the injected beam has remained small.